Solid oxidation catalysts, in particular for epoxidation of prochiral compounds

ABSTRACT

The invention concerns recyclable solid oxidation catalysts comprising a metal compound of a pentavalent or hexavalent metal M and selected among the group consisting of tantalum, vanadium, niobium, chromium, molybdenum, tungsten, grafted at the surface of a solid oxide with at least one, preferably one, covalent bond between an oxygen atom of the solid oxide and the metal M atom, the grafted metal compound having at least two alkoxy groups bound to the metal by the oxygen atom. Preferably, at least 2 alkoxy groups bound to the metal M belong to a polyol unit, preferably diol. The invention also concerns oxidation methods, in particular epoxidation methods using same.

The present invention relates to novel solid oxidation catalysts whichmake possible in particular the oxidation of prochiral compounds, inparticular the asymmetric epoxidation of prochiral olefinic doublebonds, more particularly of a carbinol compound exhibiting an ethylenicdouble bond separated from the carbinol group by 0 to 1 C, preferablythose of allyl alcohols, to their method of preparation and the use ofthese solid catalysts in epoxidation reactions.

The introduction of a chiral center onto organic molecules has quiteconsiderable industrial potentialities. This is because natural productsare normally chiral, with only one enantiomer exhibiting a usefulbiological activity. Medicaments, agrochemicals, cosmetics or moregenerally any molecule which is used in life sciences generally belongto this family of chiral compounds with one or more centers ofasymmetry. The separation of the enantiomers from a racemic mixture isexpensive, lengthy and not economically profitable. One of the solutionsenvisaged for improving this irrefutable fact was to find catalysts,which are predominantly homogeneous. These catalysts are generallytransition metal complexes which exhibit chiral ligands. Numerousenantioselective catalytic reactions exist.

In particular, the synthesis of enantiopure epoxyalcohols, used inparticular as precursors of active principles for pharmaceuticalproducts, is very important industrially (B. E. Rossiter “AsymmetricSynthesis”, Academic Press, 1985, vol. 5, pp. 193-246; M. Bulliard andW. Shum, “Proceedings of the Chiral'95 USA symposium” 1995, pp. 5-8;U.S. Pat. No. 4,764,628).

The catalysts currently known for reactions of this type are generallychiral titanium compounds used in the homogeneous liquid phase,according to the principle proposed by Katsuki and Sharpless (J. Am.Chem. Soc., 1980, 102, 5974 and U.S. Pat. No. 4,471,130), and cannot bereused (in this document, the chiral compounds can also be tantalum,zirconium, hafnium, niobium, vanadium and molybdenum compounds and thelike). See also Johnson and Sharpless “Comprehensive Organic Synthesis”,Pergamon Press, 1991, vol. 7, pp. 389-436 and “Catalytic AsymmetricSynthesis”, edited by I. Ojima, VCH, 1993, 103-158; Gao, Sharpless etal., J. Am. Chem. Soc., 1987, 109, 5765-5780. However, these catalystscannot be easily separated from the reaction medium and their separationis, in some cases, particularly harmful to the reaction yield.Furthermore, they cannot be recycled and they cannot be used in acontinuous process.

Farrall et al. (Nouv. J. Chim., 1983, 7, 449) describe a tartrategrafted onto a polystyrene resin. A titanium alkoxide was grafted ontosuch a solid and similar results but ones much inferior to thosedescribed by Sharpless et al. were obtained. Another publication byChoudary et al. (J. Chem. Soc., Chem. Commun., 1990, 1186) alsodescribed the incorporation of titanium-based Sharpless complexes in aclay of montmorillonite type. The solid proved to be active inasymmetric epoxidation but was not recycled. A publication by Adam,Corma et al. (J. Mol. Catal. A., 1997, 117, 357) reportsdiastereoselective and non-enantioselective epoxidations of allylalcohols with aqueous hydrogen peroxide solution catalyzed bytitanium-comprising zeolites. However, in this case, the starting allylalcohols are already chiral and the catalysts achiral. These catalystswere not recycled.

A. Corma et al. (J. Mol. Catal. A., 1996, 107, 225-234) have proposed amolybdenum catalyst supported in a zeolite with a chiral ligand. Itwould be possible to recycle this catalyst but its enantioselectivity islow.

An object of the present invention is to provide novel solid oxidationcatalysts which can be easily and efficiently recycled.

A more particular object of the present invention is to provideheterogeneous catalysis for the oxidation of prochiral compounds whichcombines the following properties:

performances (rate of reaction, activity per catalytic site, reactionyield and selectivity, enantiomeric excess obtained) equal to orsuperior to those of the homogeneous catalysts currently used,

ease of separation from the reaction medium,

reusable, while retaining the performances, and

optionally usable in a continuous process.

A more particular object of the invention is to provide suchheterogeneous catalysis for the asymmetric epoxidation of prochiralolefinic double bonds, in particular those of allyl alcohols.

A first subject-matter of the present invention is a solid oxidationcatalyst comprising a metal compound of a pentavalent or hexavalentmetal M, selected from the group consisting of tantalum, vanadium,niobium, chromium, molybdenum and tungsten, grafted to the surface of asolid oxide by at least one, preferably one, covalent bond between anoxygen atom of the solid oxide and the metal atom M, the grafted metalcompound exhibiting at least two alkoxy groups bonded to the metal viathe oxygen atom, preferably at least one of these alkoxy groups beingchiral.

The preferred metals are tantalum, vanadium and niobium. The mostpreferred metal is tantalum.

The alkoxy groups OR bonded to the metal M via the oxygen atom areidentical or different (different means that at least one of the Rradicals is different from the others). The R radicals are C₁ to C₃₀,preferably C₁ to C₈, more preferably still C₁ to C6, hydrocarbonaceouschains which can be aliphatic or unsaturated, optionally cyclic, inparticular aromatic, and which can optionally be functionalized, forexample by halide, alcohol or ester functional groups and the like. TheR radicals are preferably selected from methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, tert-butyl, cyclohexyl, vinyl, allyl,phenyl or trialkylsilyl (R₃Si—; R=Me, Et, i-Pr or n-Bu).

According to a preferred form of the invention, at least 2 alkoxy groupsbonded to the metal M belong to a polyol unit, in particular a triol ordiol unit, preferably a diol unit. These polyol units confer on thecatalyst a chiral group of high stability which cannot be easilydisplaced or exchanged under the effect of other molecules when thecatalyst will be employed in an epoxidation reaction.

Mention may be made, among the chiral diol units which come under thepresent invention, of:

1,2-propylene glycol

2,3-butanediol

3,4-dimethyl-3,4-hexanediol

4,5-octanediol

2,3-hexanediol

1,3-di(p-nitrophenyl)propane-1,2-diol

2,4-pentanediol

tartaric acid esters, for example:

dimethyl tartrate

diethyl tartrate

diisopropyl tartrate

distearyl tartrate

diphenyl tartrate

tartaric acid diamide

N,N-dimethyl tartaric acid diamide

trans-1,2-cyclopentanediol

diethyl 1,2-cyclohexanediol-1,2-dicarboxylate

dimethyl 2,4-dihydroxyglutarate

ethyl N,N-diethyl tartrate monoamide

2,5-dioxo-3,4-octanediol

1,2-bisacetylethylene glycol

bis-2,2′-(2-hydroxycaprolactone)

binaphthol

1,2-bis(methoxyphenyl)ethane-1,2-diol.

Diethyl or diisopropyl tartrate units are preferred.

Generally, the metal compound grafted onto the solid oxide preferablycomprises 4 alkoxy groups when the metal M is selected from tantalum,vanadium or niobium and 4 or 5 alkoxy groups when the metal is selectedfrom chromium, molybdenum or tungsten, those optionally belonging to apolyol unit being included within these values.

The oxidation catalysts according to the invention can also be definedby their process of preparation. It is possible, for the preparation ofthe catalyst, to preferably start from a complex of the metal M.

The precursor complexes of tantalum or another metal, which are used tocreate the bond between the metal M and the oxygen of the support (solidoxide), comprise appropriate identical or different ligands, at leastone of which can be substituted by an oxygen of the solid oxide, forexample an oxygen of a siloxy group in the case of silica, for theformation of at least one covalent bond between an oxygen atom of thesolid oxide and the metal atom M. The ligands can be in all or part, inparticular completely, alkoxy groups as described above, includingpolyol groups, or nonalkoxy ligands which, at a subsequent stage in thegrafting of the metal complex to the solid oxide, can be substituted byalkoxy groups. The complex can comprise more than one metal atom M butwill preferably be monoatomic for this metal. These complexes cancorrespond to the following general formula:

(MX_(a))_(b)L_(c)

where:

M is the metal selected from tantalum, vanadium, niobium, chromium,molybdenum or tungsten

a is an integer ranging from 4 to 6, it being understood that, when a=6,M is chromium, molybdenum or tungsten and it being understood that it ispossible to have a double or a triple bond

b is an integer ranging from 1 to 4

c is an integer ranging from 0 to 16

X are ligands which can be identical or different (different means thatat least one of the ligands X is different from the others) and areselected from:

the above R radicals

OR as described above with respect to the alkoxy groups,

acac (acetylacetonate)

NR₂, with R as above

halogen atom, in particular Cl, Br or I,

L is any neutral (neither anionic nor cationic) molecular ligand, forexample EtOH, NH₃, pyran, and the like.

Preferably b=1. Preferably c=0.

Mention may in particular be made, among the complexes which can beused, of those which follow:

[(CH₃)₃CCH₂]₃TA═CHC (CH₃)₃ or any compound exhibiting at least one Ta-Cbond and preferably several of these bonds, in particular a compound ofthe TaR₅ type, with R, which are identical or different, in particularidentical, as above, preferably selected from methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, tert-butyl, cyclohexyl, vinyl, allyl,phenyl or trialkylsilyl (R₃Si—; R=Me, Et, i-Pr or n-Bu), such as, forexample, Ta(Me)₅;

Ta(OC₂H₅)₅ or more generally Ta(OR)₅, with R identical or different, inparticular identical, as above:

Ta(acac) (OC₂H₅)₄ or more generally Ta(acac)_(x)(OR)_(5-x), at the samelevel with x=1 or 2 and R identical or different, in particularidentical, as above;

TaCl₅, TaBr₅ or TaI₅.

The operation is carried out in the same way with vanadium, niobium,chromium, molybdenum and tungsten, the appropriate precursors beingchosen where tantalum is replaced with the chosen metal, taking intoaccount its own valency, for example:

if M=V or Nb:

MR₅, M(OR)₅, M(acac)_(x)(OR)_(5−x), or M(halogen)₅

if M=Cr, Mo or W:

MR₆, M(OR)₆, M(acac)_(x)(OR)_(6−x) or M(halogen)₆

with x=1 or 2

with R identical or different, preferably identical, as above.

By way of examples:

WCl₆

MoCl₆

W(OEt)₆

W(CH₂C(CH₃)₃)₃(≡CC(CH₃)₃).

The preferred substrate is silica. However, other inorganic oxides canbe envisaged; for example alumina, silica/alumina, zeolites, includingsilicalites, titanium oxide, niobium oxide, tantalum oxide, mesoporoussilicas, and the like. The solid oxide, for example silica, willpreferably be such as obtained by an exhaustive heat treatment (with theintention of providing partial dehydroxylation and dehydration), forexample between 200 and 1100° C. for several hours (for example 10 to 20hours). Of course, a person skilled in the art will take care not toexceed the decomposition temperature or stability limit temperature ofthe solid oxide which he has chosen to use. For silica, the dehydrationis generally carried out between 200 and 500° C. to 800° C., preferablybetween 400 and 800° C., e.g. at 500 or at 700° C. approximately, inparticular if it is desired to obtain, in addition, the formation ofsurface siloxane bridges.

According to an advantageous form of the invention, the support, inparticular silica, can be treated, before grafting the metal complex,with organosilicon compounds. These compounds includemethylpolysiloxanes, such as hexamethyldisiloxane oroctamethylcyclotetrasiloxane, methylpolysilazanes, such ashexamethyldisilazane (the preferred), or hexamethyl-cyclotrisilazane,chlorosilanes, such as dimethyldichlorosilane, trimethylchlorosilane,methyl-vinyldichlorosilane or dimethylyinylchlorosilane, oralkoxysilanes, such as dimethyldimethoxysilane,dimethylyinylethoxysilane or trimethylmethoxysilane.

The complex can be transferred onto the solid oxide in particular bysublimation or by impregnation in solution.

In the case of sublimation, the organometallic complex in the solidstate is heated, preferably under vacuum (or under an inert gas) andunder temperature conditions which provide for its sublimation and itsmigration in the vapor state onto the solid oxide, which is preferablyitself in the pulverulent state or in the form of pellets or the like.Sublimation is in particular carried out between 50 and 150° C.,preferably at approximately 80° C. The deposition can be monitored, forexample by infrared spectroscopy. The grafting takes place by reactionof the complex with the functional groups of the support (OH, Si-O-Si,and the like). The solid will preferably be maintained at a temperaturegreater than or equal to ambient temperature.

It may be desirable to remove, by reverse sublimation, the excessunreacted complex which has simply been adsorbed at the surface of theoxide.

The grafting of the metal complex on the solid oxide can also be carriedout by impregnation, a suspension of solid oxide and the metal complexbeing brought directly into contact. The suspension is preferably formedof solid oxide in a solvent, in particular a non-polar solvent, such aspentane. The whole reaction is preferably carried out under an inertatmosphere, e.g. argon. The grafting reaction is in particular carriedout with stirring for several hours, the solid subsequently beingfiltered off, washed and dried, and stored under an inert atmosphere.

If it is desired to prepare a catalyst comprising alkoxy groups, it isadvisable to subsequently treat the solid obtained with an alcohol,selected in particular from ethanol, methanol, isopropanol and butanol,preferably with vapors of an alcohol, in particular of one of thosementioned above. The most preferred form is the treatment with ethanolvapors. The reaction can be carried out in particular in a temperaturerange extending from 25 to 150° C. for several hours, in particular atleast 5 hours. The amount of alcohol introduced into the receptaclecomprising the silica or another solid oxide modified by the metalcompound should preferably be greater than 0.1 mol of alcohol per gramof silica or other solid oxide. After the heat treatment, the solid ispreferably treated under dynamic vacuum, in particular lasting at least5 hours at 150° C. This treatment with an alcohol is not absolutelynecessary, in particular if the starting material is a precursor complexalready having alkoxy groups in order to graft it to the silica.

A subject-matter of the invention is thus the oxidation catalysts whichcan be obtained by the implementation of the process which has just beendescribed.

More particularly, the invention is targeted at a chiral solid catalystwhich makes possible in particular the oxidation of prochiral compounds,in particular the asymmetric epoxidation of prochiral double bonds,preferably those of carbinol compounds exhibiting an ethylenic doublebond which is separated by 0 to 1 carbon atoms from the carbinol group,in particular allyl and homoallyl alcohols, in order to produce chiralepoxyalcohols. The targeted reaction is an enantioselective reaction.

In this application, use is made of a solid catalyst in accordance withthe preferred form indicated above, namely comprising a polyol unit,preferably a diol unit. For its preparation, the starting materials area precursor catalyst, as defined above by its characteristics or itsprocess of preparation, preferably comprising alkoxy groups, moreparticularly having from 4 to 5 alkoxy groups, and a chiral polyol, inparticular a chiral diol, preferably selected from those mentionedabove, so as to exchange at least two OR groups by the polyol, inparticular the diol, and thus to form the polyol or diol unit connectedto the metal via oxygen atom(s).

The amount of chiral diol added should preferably be adjusted so as toobtain a diol:metal molar proportion of at least 0.5, in particular ofbetween 0.5 and 3, it being known that higher proportions may beeffective but are not essential. In the case of a tantalum catalyst,with diethyl tartrate as chiral diol, the optimal proportion is[tartrate:Ta] between 1 and 2. This makes it possible to prepare acatalyst which will result in a good epoxide yield and in anadvantageously enantiomeric excess, for example in the case of theepoxidation of allyl alcohol to glycidol or of trans-2-hexen-1-ol topropyloxirane-methanol.

This exchange or substitution of OR group or more generally X group, asdefined above, by a diol is preferably carried out in a solvent for thediol used, e.g. dichloromethane or pentane, these two solvents being,for example, well suited to the case of diethyl tartrate and ofdiisopropyl tartrate. The reaction medium comprising the diol and thesolid oxide to which the metal compound is grafted is preferably keptstirred for a sufficient time until the relevant chiral catalyst isobtained, generally more than 10 or 15 hours (up to 48 hours), themedium being maintained at low temperature, in particular less than orequal to 0° C., preferably between 0 and −20° C. approximately.

The medium obtained, comprising the catalyst, can be used as is for theepoxidation reaction. While awaiting its use, it is preferable to storethe catalyst at low temperature, as indicated, and preferably between 0and −20° C. It is also possible to filter the catalyst. In particular,for long-term storage, it is preferable to filter off the catalyst andto store it under cold conditions, in particular between 0 and −40° C.

Another subject-matter of the invention is thus this method offunctionalization of the alkoxide-comprising oxidation catalysts and thechiral solid catalysts capable of being obtained by the implementationof this method.

This chiral solid catalyst, based on tantalum or on another metalgrafted to the surface of silica or the like, is used in particular inthe epoxidation of carbinol compounds as defined above, preferably ofallyl alcohols, with organic hydroperoxides as epoxidation agents, as isfully known per se. They are usually aliphatic hydroperoxides, which maybe mono- or polyhydroperoxides, most commonly not having more than twohydroperoxy groups. Monohydroperoxides, in particular having from 1 to20 carbon atoms, and more particularly from 1 to 12 carbon atoms, remainthe commonest. Use is preferably made of hydroperoxides R″OOH, such asin particular R″=cumyl, tert-butyl, triphenylmethyl, n-butyl, methyl,ethylphenyl, pinanyl or trityl.

The enantiomeric excesses are of the same order of magnitude as thoseobtained with titanium in homogeneous catalysis. However, highlyadvantageously, the catalyst is filtered off and is reused for a freshcatalytic test and similar results are then obtained. The recycling canbe carried out several times without a significant loss in activity orin stereoselectivity. Furthermore, it has been possible to demonstratethat the tantalum or other metal does not pass into solution and thatthe solid retains its same content of tantalum or other metal afterseveral recycling operations. Surprisingly, if the same experiment iscarried out with titanium complexes, a very low activity without anenantiomeric excess is then obtained, demonstrating, if it werenecessary, the entirely unexpected nature of the invention.

Another subject-matter of the present invention is thus a method for theoxidation of prochiral compounds, in which the prochiral compound, anoxidant and a solid catalyst according to the invention are brought intocontact and are reacted together. A particular subject-matter of theinvention is a method for the asymmetric epoxidation of prochiralolefinic double bonds of a compound to be epoxidized, more particularlyof a carbinol compound exhibiting an ethylenic double bond separatedfrom the carbinol group by 0 to 1 C, preferably those of allyl alcohols,in which method the prochiral compound to be oxidized, in particular theallyl alcohol or the like, is brought into contact and is reactedtogether with a chiral solid catalyst according to the invention,comprising alkoxy groups and a group of the chiral polyol type,preferably chiral diol type, and an oxidant, in particular organichydroperoxide or hydrogen peroxide.

The present invention is targeted in particular at the asymmetricepoxidation of allyl alcohols in general, unsubstituted or substituted,including polysubstituted, by groups such as methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, tert-butyl, C_(n)H_(2n+1) alkyl groupswith n=5 to 15, cyclohexyl, vinyl, allyl, phenyl or trialkylsilyl(R₃Si—; R=Me, Et, i-Pr or n-Bu).

Mention may particularly be made, among the allyl alcohols which comeunder the present invention, of:

base allyl alcohol (2-propen-1-ol): CH₂═CHCH₂OH

allyl alcohols substituted in the 2-position: CH₂═C(R)CH₂OH

geraniol

nerol

linalol

allyl alcohols monosubstituted in the 3E-position: CH(R)═CHCH₂OH

allyl alcohols monosubstituted in the 3Z-position: CH(R)═CHCH₂OH

allyl alcohols disubstituted in the (2 and 3Z or 3E)-positions:CH(R¹)═C(R²)CH₂OH

allyl alcohols disubstituted in the (3, 3)-positions: C(R¹)(R²)═CHCH₂OH

allyl alcohols trisubstituted in the (2, 3, 3)-positions:C(R¹)(R²)═C(R³)CH₂OH

allyl alcohols monosubstituted in the 1-position: CH₂═CHCH(R)OH

allyl alcohols disubstituted in the (1, 1)-positions:

CH₂═CHC(R¹)(R²)OH

allyl alcohols disubstituted in the (1, 2)-positions: CH₂═C(R¹)CH (R²)OH

allyl alcohols disubstituted in the (1 and 3Z or 3E)-positions:CH(R¹)═CHCH(R²)OH

allyl alcohols trisubstituted in the (1, 1, 2)-positions:CH₂═C(R¹)C(R²)(R³)OH

allyl alcohols trisubstituted in the (1, 1 and 3Z or 3E)-positions:CH(R¹)═CHC(R²)(R³)OH

allyl alcohols trisubstituted in the (1, 2 and 3Z or 3E)-positions:CH(R¹)═C(R²)CH(R³)OH

allyl alcohols trisubstituted in the (1, 3, 3)-positions:C(R¹)(R²)═CHCH(R³)OH

allyl alcohols tetrasubstituted in the (1, 1, 2 and 3Z or 3E)-positions:CH(R¹)═C(R²)C(R³)(R⁴)OH

allyl alcohols tetrasubstituted in the (1, 1, 3, 3)-positions:C(R¹)(R²)═CHC(R³)(R⁴)OH

allyl alcohols tetrasubstituted in the (1, 2, 3, 3)-positions:C(R¹)(R²)═C(R³)CH(R⁴)OH

allyl alcohols pentasubstituted in the (1, 1, 2, 3, 3)-positions:C(R¹)(R²)═C(R ³)C(R⁴ )(R⁵)OH with R, R¹, R², R³, R⁴ and R⁵ selected,independently from one another, from:

methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl,C_(n)H_(2n+1) alkyl groups with n=5 to 15, cyclohexyl, vinyl, allyl,phenyl or trialkylsilyl (R₃Si—; R=Me, Et, i-Pr or n-Bu).

The substrate to be oxidized, in particular to be epoxidized, forexample allyl alcohol, is subsequently introduced into the medium(solvent+catalyst, maintained at a temperature of between +20 and −20°C., preferably between 0 and −20° C.) in a proportion such that the[substrate:metal] molar ratio is in particular between 1 and 10 000,preferably between 2 and 1 000, preferably between 4 and 200, inparticular in the case of the epoxidation of an allyl alcohol catalyzedby a solid prepared from a compound of tantalum or other metal.

Throughout the duration of the epoxidation reaction, the reaction mediumis preferably maintained between +20 and −20° C., in particular between0 and −20° C.

The solvent used for the epoxidation reaction is preferably nonpolar andcan be, for example, dichloromethane, pentane, hexane, and the like.This solvent can be distilled beforehand. It must, in any case, beemployed carefully dehydrated; for this, it can be stored over a 3 Å or4 Å zeolite sieve, itself well dehydrated beforehand (for example byheat treatment under vacuum at 300° C. for at least 15 hours). Theamount of solvent used is adjusted according to the concentration ofallyl alcohol desired in the reaction medium at the beginning of thereaction. A concentration of allyl alcohol of at least 0.1M in thesolvent is advisable. When the solvent is introduced into the reactor inorder to suspend the solid (supported metal compound) a dehydratingagent, such as zeolite 3 Å, preferably as a powder and well dehydratedbeforehand, can be added to the reaction medium.

The oxidant is introduced slowly into the reaction medium. Theepoxidation agents used are described above and are advantageouslyorganic hydroperoxides ROOH, such that R=cumyl, tert-butyl,triphenylmethyl, α-phenylbenzyl, α, α′-methylphenyl-benzyl, pinanyl,n-butyl or methyl, and optionally hydrogen peroxide H₂O₂, preferably inan anhydrous medium. These oxidants are preferably carefully dehydratedbefore being introduced into the reaction medium, for example overzeolite. It is preferable for the [oxidant:substrate to be epoxidized]molar ratio to be greater than 1 in order to obtain as great aconversion as possible of the substrate to epoxide. In the examplesmentioned below, the value of this [oxidant:substrate to be epoxidized]ratio is approximately 2.

The mixture is subsequently left to stir, in particular for 4 to 48hours, while keeping the temperature constant, e.g. at a value fixedbetween +20 and −20° C. The reaction medium is subsequently filtered inorder to collect, on the one hand, the solid, and, on the other hand,the filtrate. The solid is washed several times with the solvent and theliquid phases are combined together. The product can then be isolatedand the solvent recycled.

It should be noted that, in the present application, proportions orratios refers to molar proportions or molar ratios.

The invention will now be described in more detail using embodimentstaken as non-limiting examples.

EXAMPLES Example 1 (Comparative Example)

This example illustrates the fact that the deposition of titaniumalkoxide on a silica heat treated beforehand at a temperature of 500° C.under dynamic vacuum (silica 500) does not result in a catalyst whichcomes under the present invention, whereas a titanium alkoxide used inhomogeneous medium results in a catalyst which is effective for theepoxidation of allyl alcohol to glycidol.

1-a A mass of 2.174 g of Degussa “Aerosil® 200” silica, wettedbeforehand with water and then dried, is treated in a hermeticallysealed Schlenk tube under dynamic vacuum (P<5×10⁻⁵ mbar) at atemperature of 500° C. for 15 hours. After returning this powder (silica500) to ambient temperature, 20 ml of thoroughly dry n-pentane,distilled beforehand over sodium/potassium amalgam and stored underargon in the presence of zeolite 4 Å, followed by 251 mg of titaniumtetraisopropoxide, are carefully introduced into the Schlenk tubewithout re-exposing to the air and under an argon atmosphere. Thismixture is stirred for 4 hours at ambient temperature. A reaction takesplace with the emission of isopropanol, demonstrated by gaschromatography. The mixture is subsequently filtered through sinteredglass, still carefully under an argon atmosphere, and the solid thusrecovered is washed four times with 10 ml of thoroughly dehydratedpentane and then dried under dynamic vacuum at ambient temperature for15 hours and then at 60° C. for 2 hours. This solid is subsequentlystored under argon. Chemical analysis shows that it comprises contentsby mass of 1.8% of titanium and of 3.03% of carbon, which corresponds toa C/Ti molar ratio of 6.7.

A mass of 215 mg of this solid (81 μmol of Ti) is subsequently withdrawnand placed under argon in a 100 ml round-bottomed flask in which theepoxidation reaction will subsequently be carried out. Afterwards, 1 gof zeolite 3 Å, dehydrated beforehand at 300° C., and then 10 ml ofcarefully dehydrated dichloromethane are introduced and this mixture ismaintained at 0° C. 0.1 ml of a 1.0M solution of diisopropyl(+)-tartrate (100 μmol) in dichloromethane is subsequently added and themixture is stirred for 15 hours at 0° C. An amount of 97 mg of allylalcohol (1.67 mmol) is then introduced, which corresponds to a Ti/allylalcohol molar ratio of 5/100. Then, 30 minutes later, 0.74 ml of 80%technical grade cumyl hydroperoxide (Aldrich), dehydrated beforehandover 3 Å sieve (approximately 4.0 mmol), is slowly added over 30minutes. The combined mixture is left to stir for 48 hours at 0° C. Thereaction medium is subsequently filtered in order to recover, on the onehand, the solid and, on the other hand, the filtrate. The solid iswashed five times with the solvent and the liquid phases are combinedtogether. An amount of 40 mg of n-dodecane (235 μmol), used as referencecompound, is added thereto and the filtrate is analyzed by gaschromatography (GC) on a cyclodextrin phase chiral column (Lipodex Efrom Macherey-Nagel). This method gives the glycidol yield, 14% withrespect to the starting amount of allyl alcohol charged, the degree ofconversion of the allyl alcohol, 17%, and thus also the selectivity ofthe reaction, 82%, and the enantiomeric excess, approximately 9%(predominantly R-glycidol). As the amount of glycidol formed is verylow, the product was not isolated in this case. This shows that thissilica, modified by supported titanium, is not a catalyst which issufficiently active for this reaction. Quantitative determination of thecontent of titanium in the solid by elemental chemical analysis afterits use as catalyst, followed by washing with dichloromethane a numberof times at ambient temperature, showed that there was no loss oftitanium from the solid (1.8% of Ti by mass).

1-b The same reaction was repeated but this time it was carried out in ahomogeneous medium, using 470 mg of titanium tetraisopropoxide (1.68mmol) instead of the solid prepared by impregnation with silica withthis same compound (Example 1-a), 2 g of powdered zeolite 3 Å, 64 ml ofdichloromethane and 2.0 ml of a 1.0M solution of diisopropyl(+)-tartrate (2.0 mmol) in dichloromethane. The combined mixture isstirred in a round-bottomed flask for 4 hours at 0° C. 1.86 g of allylalcohol (32 mmol) and then, 30 minutes later, 11.5 ml of 80% technicalgrade cumyl hydroperoxide (Aldrich), dried beforehand over 3 Å sieve(approximately 64 mmol), are subsequently introduced successively. Thiscorresponds to a Ti/allyl alcohol molar ratio of 5/100. The combinedmixture is left to stir for 48 hours at 0° C. in order for theepoxidation reaction to take place. Glycidol is then produced with yieldof 72%, a selectivity of 95% and an enantiomeric excess of 80%(predominantly S-glycidol).

Example 2 (comparative example)

This example illustrates the unexpected fact that a tantalum alkoxidedeposit on silica 500, followed by treatment with diisopropyl tartrate,results in a catalyst which is active in asymmetric epoxidation,whereas, in a homogeneous medium, tantalum pentaethoxide is not aprecursor of an active species.

2-a A mass of 1.26 g of silica 500 is placed in a Schlenk tube equippedwith a pig-tail store comprising approximately 500 mg of tantalumpentaethoxide under vacuum. This store is opened and the tantalumcompound is sublimed in order to be brought into contact with thesilica. The grafting reaction is carried out by heating the silica andthe tantalum pentaethoxide at 120° C. for 3 hours. The excess tantalumcompound is subsequently removed by carefully washing the solid, whichhas not been re-exposed to the air, on a sintered glass under argonatmosphere four times with 20 ml of thoroughly dehydrated pentane. Thesolid collected by filtration is dried under vacuum (5×10⁻⁵ mbar) for 20hours at 60° C. Chemical analysis of the solid gives a content by massof tantalum of 4.87% and a C/Ta atomic ratio of 8.9; by ¹³C NMR of thesolid, two peaks are observed exhibiting chemical shifts ofapproximately 18 and 70 ppm, attributed respectively to the carbon atomsin the positions γ and β to the tantalum atom in a structure of theTa—O—C_((β))H₂—C_((γ))H₃ type firmly grafted to the surface of thesilica.

A mass of 228 mg of this solid (62 μmol Ta) is placed in a 50 mlround-bottomed flask under an argon atmosphere and 3 ml ofdichloromethane are added. The suspension is cooled to 0° C. and 70 μlof a 1.0M solution of diisopropyl (+)-tartrate (70 μmol) are addedthereto. The combined mixture is left to stir under an argon atmospherefor 48 hours at 0° C. An mount of 17.4 mg of allyl alcohol (300 μmol) issubsequently introduced and the combined mixture is left to stir for 30minutes. An amount of 0.1 ml of a 6.0M solution of TBHP indichloromethane (600 μmol) is added and the solution is left to stirunder argon for 48 hours at 0° C. A glycidol yield of 48% and a degreeof conversion of the allyl alcohol of 52%, thus a selectivity of 92%,for an enantiomeric excess of 83% (predominantly S-glycidol) areobtained.

2-b The same reaction was repeated but this time it was carried out in ahomogeneous medium, using 111 mg of tantalum pentaethoxide (274 μmol)instead of the solid prepared by sublimation of this same compound ontosilica (Example 2-a), with 2 g of powdered zeolite 3 Å, 130 mldichloromethane and 0.3 ml of a 1.0M solution of diisopropyl(+)-tartrate (300 μmol) in dichloromethane. The combined mixture isstirred in a 250 ml round-bottomed flask for 4 hours at 0° C. 794 mg ofallyl alcohol (13.7 mmol) and then, 30 minutes later, 4.5 ml of a 6.0Msolution of TBHP in dichloromethane (27 mmol), which solution is driedbeforehand over 3 Å sieve, are subsequently introduced successively.This corresponds to a Ta/allyl alcohol molar ratio of approximately2/100. The combined mixture is left to stir for 48 hours at 0° C. inorder for the epoxidation reaction to take place. The glycidol isproduced with a very low yield of 0.4%, for a conversion of the allylalcohol of 0.5% and with an enantiomeric excess of 45% (predominantlyR-glycidol, in contrast to Example 2-a using a solid catalyst).

Example 3

This example shows that a slight change in the experimental conditions(in this instance, essentially a change in the concentration of theallyl alcohol in the medium, which is approximately 1.0M instead of 0.1Min Example 2-a) can result in an improvement in the enantiomeric excess.

A mass of 198 mg of a solid, prepared as in example 2-a and exhibiting acontent by mass of tantalum of 2.79% (29 μmol Ta), is placed in a 50 mlround-bottomed flask under an argon atmosphere and 2.74 ml ofdichloromethane are added. The suspension is cooled to 0° C. and 70 μlof 1.0M solution of diisopropyl (+)-tartrate (70 μmol) are addedthereto. The combined mixture is left to stir under an argon atmospherefor 48 hours at 0° C. An amount of 165.9 mg of allyl alcohol (2.86 mmol)is subsequently introduced and the combined mixture is left to stir for30 minutes. An amount of 1.14 ml of a 5.0M solution of TBHP indichloromethane (5.7 mmol) is added and the solution is left to stirunder argon for 24 hours at 0° C. A glycidol yield of 34% and a degreeof conversion of the allyl alcohol of 45%, thus a selectivity of 76%,for an enantiomeric excess of 98.5% (predominantly S-glycidol) areobtained.

Example 4

This example illustrates the fact that the catalyst of Example 2 can beprepared by other routes which result in an even more active catalyst.

4-a The carbene compound used for the deposition of tantalum, of formula((CH₃)₃CCH₂)₃Ta═CHC(CH₃)₃, was synthesized according the proceduredescribed by Schrock et al. (J. Am. Chem. Soc., 1978, 100, 3359). A massof approximately 100 mg of this compound is carefully transferred underargon into a store terminating in a “pig-tail”. This store is connectedto a glass Schlenk tube comprising 700 mg of silica 500. The carbenecompound is brought into contact with the silica by breaking thepig-tail of the store and the complex is sublimed towards the silica byheating at 80° C. (the temperature of the solid must be less than 100°C.). The gases given off during the reaction are cryogenically trappedand then analyzed by GC. The excess organometallic compound is removedby reverse sublimation and then the store comprising the unreactedcomplex is taken out by sealing it off. Ethanol vapors are introducedinto the reactor (the total amount of ethanol introduced must be greaterthan 0.1 mol of ethanol per gram of silica). The combined mixture isleft, the solid being heated for 15 hours at 150° C. The excess alcoholis then removed by placing the reactor under vacuum for 12 hours whilemaintaining the temperature of the solid at 150° C. Chemical analysisgives a content with a mass of tantalum of 3.36% (C/Ta=12); by ¹³C NMRof the solid, two peaks are observed, as above, exhibiting chemicalshifts of approximately 18 and 70 ppm.

An amount of 134 mg (25 μmol of Ta) of this solid is placed in a 50 mlround-bottomed flask and 10 ml of dichloromethane are added. Thecombined mixture is cooled to 0° C. and 26 μl of a 1.0M solution ofdiisopropyl (+)-(R,R)-tartrate (26 μmol) are introduced. The suspensionis left to stir at this temperature for 15 hours, 65 mg of allyl alcohol(1.124 mmol) are then introduced and the mixture is left for 30 minbefore introducing 0.4 ml of a 6.0M solution of TBHP in dichloromethane(approximately 2.4 mmol). The medium is left for 48 hours at 0° C. inorder for the epoxidation reaction to take place. It is subsequentlyfiltered through a sintered glass under argon and the solid catalyst iswashed four times with dichloromethane at ambient temperature and thesolutions are combined together. A known amount of n-C₁₂ is subsequentlyadded. GC analysis gives a glycidol yield of 31%, a conversion of theallyl alcohol of 32%, i.e. a selectivity of 97%, and an enantiomericexcess of 85% (predominantly S-glycidol). Chemical analysis gives acontent by mass of tantalum of 3.30% for the solid used and washed. Thesolution does not comprise detectable amounts of tantalum (<5 ppm). Thisshows that the tantalum does not pass into solution during the reaction.

4-b The carbene compound ((CH₃)₃CCH₂)₃Ta═CHC(CH₃)₃ can also be graftedonto the silica by impregnation in a solvent instead of sublimation inthe gas phase (cf. Example 3-a). A mass of 2.1 g of silica 500 is placedin an approximately 250 ml Schlenk tube. A solution comprising 350 mg ofthe compound ((CH₃)₃CCH₂)₃Ta═CHC(CH₃)₃ (0.75 mmol) dissolved in 20 ml ofcarefully dehydrated pentane is added thereto dropwise via a droppingfunnel. The mixture is subsequently left to stir for one hour at ambienttemperature. The excess organometallic compound is removed by reversesublimation. Chemical analysis gives a content by mass of tantalum of5.40% and a C/Ta molar ratio of 13. The amount of neopentane given offduring the impregnation was measured by GC and an NpH/Ta molar ratio of1.52 is obtained. The treatment under ethanol vapors is carried out asin the preceding example. Release of neopentane takes place, the amountof which was also measured by GC, and an NpH/Ta molar ratio of 2.50 isobtained. Thus, overall, 4.0 mol of neopentane were emitted per mole oftantalum grafted during the preparation, which corresponds to the lossof the three neopentyl ligands and of the neopentylidine ligand situatedaround the tantalum atom in the starting compound((CH₃)₃CCH₂)₃Ta═CHC(CH₃)₃ used. The excess alcohol is removed as aboveby placing the sample under vacuum (P<5×10⁻⁵ mbar) for 30 minutes at150° C. Elemental analysis shows that the solid comprises 5.40% oftantalum and a C/Ta molar ratio of 7.2 is determined; by ¹³C NMR of thesolid, two peaks are observed, as above, exhibiting chemical shifts ofapproximately 18 and 70 ppm.

An amount of 254 mg (76 μmol of Ta) of this solid is placed in a 100 mlround-bottomed flask with approximately 500 mg of thoroughly dehydratedpowdered zeolite 3 Å, and 38 ml dichloromethane are added. The combinedmixture is cooled to 0° C. and 84 μl of a 1.0M solution of diisopropyl(+)-(R,R)-tartrate (84 μmol) in dichloromethane are introduced. Thesuspension is left to stir at this temperature for 15 hours, then 228 mgof allyl alcohol (3.8 mmol) are introduced and the mixture is left for30 min before introducing 1.0 ml of 6.5M solution of TBHP (tBuOOH) indichloromethane (approximately 6.5 mmol). The medium is left for 48 oursat 0° C. in order for the epoxidation reaction to take place. It issubsequently filtered through a sintered glass under argon and the solidcatalyst is washed four times with dichloromethane at ambienttemperature and the solutions are combined together. A known amount ofn-Cl₁₂ is subsequently added. Analysis by GC gives a glycidol yield of56%, a conversion of the allyl alcohol of 60%, i.e. a selectivity of93%, and an enantiomeric excess of 85% (predominantly S-glycidol).

Example 5

A mass of 261 mg of a solid exhibiting a content by mass of tantalum of4.92% (72 μmol of Ta), prepared as indicated in Example 4-b, is placedin a 50 ml round-bottomed flask. 3.8 ml of dichloromethane aresubsequently added and the combined mixture is cooled to 0° C., and then75 μl of a 1.0M solution of diisopropyl (+)-(R,R)-tartrate (75 μmol) areintroduced. The suspension is left to stir at this temperature for 15hours, then 22.8 mg of allyl alcohol (0.38 mmol) are introduced and themixture is left to stir for 30 min before introducing 0.15 ml of a 6.5Msolution of TBHP in dichloromethane (approximately 0.97 mmol). Theconcentration of allyl alcohol in the solution is 0.094M. The medium isleft for 48 hours at 0° C. in order for the epoxidation reaction to takeplace. It is subsequently filtered through a sintered glass under argonand the solid catalyst is washed four times with dichloromethane atambient temperature and the solutions are combined together. Analysis byGC gives a glycidol yield of 77%, a conversion of the allyl alcohol of79%, i.e. a selectivity of 98%, and an enantiomeric excess of 84%(predominantly S-glycidol).

Example 6

An amount of 258 mg of a solid, prepared as in Example 4-b andexhibiting a content by mass of tantalum of 5.63% (80 μmol of Ta), isplaced in a 50 ml round-bottomed flask and 9 ml dichloromethane areadded. The combined mixture is cooled to 0° C., and 0.1 ml of a 1.0Msolution of diisopropyl (+)-(R,R)-tartrate (100 μmol) is introduced. Thesuspension is left to stir at this temperature for 15 hours, then 227 mgof allyl alcohol (3.91 mmol) are introduced and the mixture is left for30 min before introducing 1 ml of a 5.0M solution of TBHP indichloromethane (approximately 5 mmol). The Ta/allyl alcohol ratio isapproximately 2/100 and the concentration of allyl alcohol in thesolution is 0.38M. The medium is left for 48 hours at 0° C. in order forthe epoxidation reaction to take place. It is subsequently filteredthrough a sintered glass under argon and the solid catalyst is washedfour times with dichloromethane at ambient temperature and the solutionsare combined together. A known amount of n-Cl₁₂ is subsequently added.Analysis by GC gives a glycidol yield of 19.5%, a conversion of theallyl alcohol of 20%, i.e. a selectivity of 98%, and an enantiomericexcess of 94% (predominantly S-glycidol).

Example 7

An amount of 508 mg of a solid, prepared as in Example 4-b andexhibiting a content by mass of tantalum of 4.92% (138 μmol of Ta), isplaced in a 150 ml round-bottomed flask and 65 ml of dichloromethane areadded. The combined mixture is cooled to 0° C., and 0.15 ml of a 1.0Msolution of diisopropyl (+)-(R,R)-tartrate (150 μmol) is introduced. Thesuspension is left to stir at this temperature for 15 hours, then 400 mgof allyl alcohol (6.9 mmol) are introduced and the mixture is left for30 min before introducing 2 ml of a 6.5M solution of TBHP indichloromethane (approximately 13 mmol). The concentration of allylalcohol in the solution is 0.10M. The medium is left for 48 hours at 0°C. in order for the epoxidation reaction to take place. It issubsequently filtered through a sintered glass under argon and the solidcatalyst is washed four times with dichloromethane at ambienttemperature and the solutions are combined together. A known amount ofn-C₁₂ is subsequently added. Analysis by GC gives a glycidol yield of30%, a conversion of the allyl alcohol of 31%, i.e. a selectivity of97%, and an enantiomeric excess of 84% (predominantly S-glycidol).

Example 8

This example shows that, by using diisopropyl (−)-tartrate as chiralinductor instead of the diisopropyl (+)-tartrate which was employed inthe preceding examples, the predominant product is the other enantiomerof glycidol, R-glycidol.

The reaction is carried out exactly as in Example 7 but this time usinga 1.0M solution of diisopropyl (−)-(S,S)-tartrate. Analysis by GC givesa glycidol yield of 29%, a conversion of the allyl alcohol of 30%, i.e.a selectivity of 97%, and an enantiomeric excess of 83% (predominantlyR-glycidol).

Example 9

This example shows that, by carrying out the reaction at a temperatureof 20° C. instead of 0° C., different results are obtained with, inparticular, a poorer enantiomeric excess.

The reaction is carried out exactly as in Example 7 but the medium isthis time maintained at 20° C. throughout the experiment. It issubsequently filtered through a sintered glass under argon, the solidcatalyst is washed four times with dichloromethane at ambienttemperature and the solutions are combined together. A known amount ofn-C₁₂ is subsequently added. Analysis by GC gives a glycidol yield of35%, a conversion of the allyl alcohol of 45%, i.e. a selectivity of78%, and an enantiomeric excess of 50% (predominantly S-glycidol).

Example 10

This example shows that it is possible to use a Ta/allyl alcohol molarratio of less than 2/10 000 instead of approximately 2/100 in the aboveexamples.

An amount of 14 mg of a solid, prepared as in Example 4-a and exhibitinga content by mass of tantalum of 5.0% (3.9 μmol of Ta), is placed in a100 ml round-bottomed flask and 50 ml of dichloromethane are added. Thecombined mixture is cooled to 0° C. and 5 μl of a 1.0M solution ofdiisopropyl (+)-(R,R)-tartrate (5 μmol) are introduced. The suspensionis left to stir at this temperature for 4 hours, then 8.5 ml of a 6Msolution of TBHP (tBuOOH) in dichloromethane (approximately 51 mmol) areintroduced and the mixture is left to stir for 30 min before introducing1.42 g of allyl alcohol (24.5 mmol). The Ta/allyl alcohol molar ratio isthus approximately 2/12 500 and the concentration of allyl alcohol inthe solution is 0.41M. The medium is left for 52 hours at 0° C. in orderfor the epoxidation reaction to take place. It is subsequently filteredthrough a sintered glass under argon and the solid catalyst is washedfour times with dichloromethane at ambient temperature and the solutionsare combined together. A known amount of n-C₁₂ is subsequently added.Analysis by GC gives a glycidol yield of 9%, a conversion of the allylalcohol of 90%, i.e. a selectivity of 100%, and an enantiomeric excessof 80% (predominantly S-glycidol).

Example 11

This example shows that the epoxidation reaction can be carried out andis highly enantioselective with virtually pure allyl alcohol, withoutany other solvent.

An amount of 240 mg of a solid, prepared as in Example 4-b andexhibiting a content by mass of tantalum of 5.63% (75 μmol of Ta), isplaced in a 50 ml round-bottomed flask and 5 ml of dichloromethane areadded. The combined mixture is cooled to 0° C. and 80 μl of a 1.0Msolution of diisopropyl (+)-(R,R)-tartrate (80 μmol) are introduced. Thesuspension is left to stir at this temperature for 15 hours and then thesolvent is evaporated under vacuum. 4.45 g of allyl alcohol (76.72 mmol)are added to this dry residue and the mixture is left for 30 min beforeintroducing 12 ml of 6.0M solution of TBHP in dichloromethane(approximately 72 mmol). The medium is left for 48 hours at 0° C. inorder for the epoxidation reaction to take place. It is subsequentlyfiltered through a sintered glass under argon and the solid catalyst iswashed four times with dichloromethane at ambient temperature and thesolutions are combined together. A known amount of n-C₁₂ is subsequentlyadded. Analysis by GC gives a glycidol yield of 5.2%, a conversion ofthe allyl alcohol of 5.5%, i.e. a selectivity of 95%, and anenantiomeric excess of 93% (predominantly S-glycidol).

The various results relating to the asymmetric epoxidation of allylalcohol to glycidol are presented in Table 1.

Conversion Selectivity % Enantiomeric M/allyl alcohol Reaction of theallyl Glycidol for excess^(c) Rotation Catalyst^(a) (M = Ta or Ti)conditions^(b) alcohol yield glycidol (predominantly) number^(d)Homogeneous 2/100 0° C. 0.5%  0.4%  80% −45% (R) 0.2 Ta(OEt)₅ (Example2-b) (+)-DIPT with zeolite TBHP Sublimed [Ta] 2/12 500 0° C.  9%  9%100%  80% (S) 560 5.5% (Example 10) (+)-DIPT without zeolite TBHPSublimed [Ta] 2/100 0° C. 32% 31% 97% 85% (S) 15 3.36% (Example 4-a)(+)-DIPT without zeolite TBHP Impregnated 2/100 0° C. 31% 30% 97% 84%(S) 15 [Ta] 4.92% (Example 7) (+)-DIPT without zeolite TBHP Impregnated2/100 0° C. 30% 29% 97% −83% (R) 15 [Ta] 4.92% (Example 8) (−)-DIPTwithout zeolite TBHP Impregnated 2/100 0° C. 20% 19.5%   98% 94% (S) 10[Ta] 5.63% (Example 6) (+)-DIPT without zeolite TBHP Impregnated 2/1000° C. 60% 56% 93% 85% (S) 30 [Ta] 5.40% (Example 4-b) (+)-DIPT withzeolites TBHP Impregnated 19/100 0° C. 79% 77% 98% 84% (S) 4 [Ta] 4.92%(Example 5) (+)-DIPT without zeolite TPHP Impregnated 1/1 028 0° C.5.5%  5.2%  95% 93% (S) 53 [Ta] 5.63% (Example 11) (+)-DIPT withoutzeolite without solvent TBHP Impregnated 2/100 20° C.  45% 35% 78% 50%(S) 18 [Ta] 5.40% (Example 9) (+)-DIPT without zeolite TBHP Sublimed1/100 0° C. 45% 34% 76% 98.5% (S) 34 Ta(OEt)₅ 2.79% (Example 3) (+)-DIPTwithout zeolite TBHP Sublimed 20/100 0° C. 52% 50% 96% 83% (S) 2.5Ta(OEt)₅ 4.87% (Example 2-a) (+)-DIPT without zeolite TBHP Homogeneous5/100 0° C. 76% 72% 95% 80% (S) 14 Ti(OiPr)₄ (Example 1-b) (+)-DIPT withzeolites CHP Key to Table 1 ^(a)The catalysts are prepared from tantalumcompounds grafted to silica 500 either by impregnation in pentane or bysublimation of the compound under vacuum. [Ta] denotes the compound((CH₃)₃CCH₂)₃Ta═CHC(CH₃)₃. Two tests in a homogeneous medium arementioned, from Ta(OEt)₅ and Ti(OiPr)₄. ^(b)Reaction temperature -Chiral inductor ((+) - DIPT = DiIsoPropyl (R,R)-Tartrate) - Presence orabsence of zeolite - Oxidant (TBHP = tert-butyl hydroperoxide or CHP =cumyl hydroperoxide) ^(c)Enantiomeric excess (e.e.) = [(S) − (R)]/[(S) +(R)]; with (S): amount of S-glycidol and (R): amount of R-glycidol whichare obtained at the end of the reaction. ^(d)Rotation number: number ofallyl alcohol molecules converted to glycidol per Ta atom.

Example 11a

This example shows that a modification to the heat treatment of theoxide support can make it possible to improve the performance of thecatalyst. In this case, an Aerosil silica treated under vacuum at 700°C. (silica 700) is used as support instead of the silica 500 of thepreceding examples. The example is carried out as in Example 4-b butstarting from a mass of 3 g of silica 700, to which a solutioncomprising 500 mg of the compound ((CH₃)₃CCH₂)₃Ta═CHC(CH₃)₃ (1.08 mmol)dissolved in 30 ml of pentane is added dropwise. The amount ofneopentane given off during the impregnation was measured by GC and anNpH/Ta_(grafted) molar ratio of 1.05 is obtained. The treatment underethanol vapors is carried out as described above. An evolution ofneopentane takes place, the amount of which was also measured by GC, andan NpH/Ta_(grafted) molar ratio of 2.95 is obtained. Thus, overall, 4.0mol of neopentane were clearly emitted per mole of tantalum whichgrafted during the preparation. The excess alcohol is removed as aboveby placing under vacuum. Elemental analysis shows that the solidcomprises 5.63% of tantalum and a C/Ta molar ratio of 8.0 is determined;by ¹³C NMR of the solid, two peaks are observed, as above, exhibitingchemical shifts of approximately 18 and 70 ppm.

An amount of 57 mg (18 μmol of Ta) of this solid is placed in a 50 mlround-bottomed flask with 15 ml of dichloromethane. The combined mixtureis cooled to 0° C. and 22 μl of a 1.0M solution of diisopropyl(+)-(R,R)-tartrate (22 μmol) in dichloromethane are introduced. Thesuspension is left to stir at this temperature for 15 hours, then 90 mgof allyl alcohol (1.5 mmol) are introduced and the mixture is left for30 min before introducing 0.46 ml of a 6.5M solution of TBHP (t-BuOOH)in dichloromethane (approximately 3 mmol) After 48 hours at 0° C. andthen separation of the catalyst by filtration, analysis by GC gives aglycidol yield of 61%, a conversion of the allyl alcohol of 62%, i.e. aselectivity of 97%, and an enantiomeric excess of 87% (predominantlyS-glycidol).

Example 11b

This example shows that the treatment of the oxide support with anothercompound, such as hexamethyldisilazane (HMDS), before the grafting ofthe tantalum can also make it possible to improve the performances ofthe catalyst. In this case, the reaction is carried out as in Example4-b; the support is an Aerosil silica treated under vacuum at 500° C.(silica 500), a mass of 4 g of which has been treated with 50 ml of10⁻²M solution of HMDS in pentane under argon. The pentane issubsequently selectively removed by placing under vacuum. This thengives a novel support, a silica 500 impregnated with HMDS, which is usedfor the preparation of the catalyst by impregnation of the tantalumcompound ((CH₃)₃CCH₂)₃Ta═CHC(CH₃)₃. This impregnation and the treatmentunder ethanol vapor are subsequently carried out as indicated in Example4-b.

An amount of 75 mg (22 μmol of Ta) of the solid thus obtained is placedin a 50 ml round-bottomed flask with 10 ml of dichloromethane. Thecombined mixture is cooled to 0° C. and 23 μl of a 1.0M solution ofdiisopropyl (+)-(R,R)-tartrate (23 μmol) in dichloromethane areintroduced. The suspension is left to stir at this temperature for 15hours, then 90 mg of allyl alcohol (1.16 mmol) are introduced and themixture is left for 30 min before introducing 0.35 ml of a 6.5M solutionof TBHP (t-BuOOH) in dichloromethane (approximately 2.3 mmol). After 48hours at 0° C. and then separation of the catalyst by filtration,analysis by GC gives a glycidol yield of 65%, a conversion of the allylalcohol of 62%, i.e. a selectivity of 95%, and an enantiomeric excess of84% (predominantly S-glycidol).

Example 11c

In this example, it is shown that the catalyst can be used in varioussolvents while exhibiting good performance. The solid described in thepreceding example (11b) is used under identical conditions forcatalyzing the epoxidation of 2-propen-1-ol by using, as solvent,dichloromethane or pentane or toluene. The results obtained arementioned in the table below:

Solvent CH₂Cl₂ Pentane Toluene Conversion of the 2-propen-1-ol 66% 55%30% Glycidol yield 64% 50% 28% Selectivity of the reaction 98% 91% 93%Enantiomeric excess 86% 88% 89%

Example 12

This example and those which follow show that the catalysts described inthe preceding examples apply not only to allyl alcohol itself but alsoto numerous other alcohols.

An amount of 500 mg of the solid prepared in Example 4-b, exhibiting acontent by mass of tantalum of 5.40% (149 μmol of Ta) is placed in a 100ml round-bottomed flask with 40 ml of solvent (CH₂Cl₂). The combinedmixture is cooled to −20° C., 31 mg of diethyl (+)-tartrate (160 μmol)are introduced and the mixture is left to stir at −20° C. for 15 hours.400 mg of trans-2-hexen-1-ol (4.0 mmol) are then added and, 30 minuteslater, 1.6 ml of a 6.0M solution of TBHP in dichloromethane (9.6 mmol).The medium is left at −20° C. for 48 hours in order for the epoxidationreaction to take place. The medium is filtered through a sintered glassunder argon, the solid catalyst is washed four times withdichloromethane at ambient temperature and the solutions are combinedtogether. Analysis by GC gives a propyloxiranemethanol yield of 34%, aconversion of the trans-2-hexen-1-ol of 35%, i.e. a selectivity of 97%,and an enantiomeric excess of 89% (predominantly(S,S)-propyloxiranemethanol).

Example 13

This example illustrates the fact that the catalyst described in Example12 can be recycled without a significant loss in activity.

The solid catalyst of Example 12, after having been washed four timeswith dichloromethane on a sintered glass at the end of the reaction (1stuse), as was mentioned, is reused for a fresh reaction, the conditionsof Example 12 being exactly reproduced. The results are then verysimilar to those obtained during the first use, with apropyloxiranemethanol yield of 31%, a conversion of thetrans-2-hexen-1-ol of 35%, i.e. a selectivity of 89%, and anenantiomeric excess of 93% (predominantly (S,S)-propyloxiranemethanol).

Example 14 (Comparative Example)

This example shows that the reaction for the epoxidation oftrans-2-hexen-1-ol to propyloxiranemethanol is more selective withcatalysis by a supported tantalum compound (Example 14-a) than withhomogeneous catalysis from the compound Ta(OEt)₅ (Example 14-b). Thecase of homogeneous catalysis using a titanium complex Ti(OiPr)₄ isgiven by way of comparison (Example 14-c).

14-a An amount of 250 mg of the solid prepared in Example 4-b,exhibiting a content by mass of tantalum of 5.40% (75 μmol of Ta), isplaced in a 50 ml round-bottomed flask with approximately 150 mg ofthoroughly dehydrated powdered zeolite 3 Å, and 18 ml of dichloromethaneare added. The combined mixture is cooled to −20° C. and 84 μl of 1.0Msolution of diethyl (+)-(R,R)-tartrate (84 μmol) are introduced. Thesuspension is left to stir at this temperature for 15 hours, then 188 mgof trans-2-hexen-1-ol (1.9 mmol) are introduced and the mixture is leftto stir for 30 min before introducing 0.6 ml of a 6.5M solution of TBHP(tBuOOH) in dichloromethane (approximately 3.9 mmol). The medium is leftfor 48 hours at −20° C. in order for the epoxidation reaction to takeplace. It is subsequently filtered through a sintered glass under argonand the solid catalyst is washed four times with dichloromethane atambient temperature and the solutions are combined together. Analysis byGC gives a propyloxiranemethanol yield of 40%, a conversion of thetrans-2-hexen-1-ol of 48%, i.e. a selectivity of 83%, and anenantiomeric excess of 90% (predominantly (S,S)-propyloxiranemethanol).

14-b The same reaction was repeated but this time it was carried out ina homogeneous medium and by using 114 mg of tantalum pentaethoxide (281μmol) instead of the solid prepared by sublimation of this same compoundonto silica, with approximately 250 mg of powdered zeolite 3 Å, 50 ml ofdichloromethane and 0.3 ml of a 1.0M solution of diethyl(+)-(R,R)-tartrate (300 μmol) in dichloromethane. The combined mixtureis stirred in a 250 ml round-bottomed flask for 4 hours at −20° C. 535mg of trans-2-hexen-1-ol (5.35 mmol) and then, 30 minutes later, 2.5 mlof a 5.0M solution of TBHP in dichloromethane (12.5 mmol), whichsolution is dried beforehand over a 3 Å sieve, are subsequentlyintroduced successively. This corresponds to a Ta/allyl alcohol molarratio of approximately 5/100. The combined mixture is left to stir for48 hours at −20° C. in order for the epoxidation reaction to take place.Analysis by GC gives a propyloxiranemethanol yield of 23%, a conversionof the trans-2-hexen-1-ol of 50%, i.e. a selectivity of 46%, and anenantiomeric excess of 55% (predominantly (R,R)-propyloxiranemethanol),in contrast to Example 14-a using a solid catalyst, for which it is the(S,S) isomer of propyloxiranemethanol which is selectively obtained).

14-c The procedure indicated in the preceding example (14-b) wasrepeated but by carrying out the reaction with Ti(OiPr)₄(253 μmol)instead of Ta(OEt)₅. After reaction, analysis of the solution by GCgives a propyloxiranemethanol yield of 80%, a conversion of thetrans-2-hexen-1-ol of 99%, i.e. a selectivity of 81%, and anenantiomeric excess of 96% (predominantly (S,S)-propyloxiranemethanol).

Example 15

A mass of 230 mg of the solid prepared as indicated in Example 2-a,exhibiting a content by mass of tantalum of 4.87% (62 μmol of Ta), isplaced in a 50 ml round-bottomed flask under an argon atmosphere and 15ml of dichloromethane are added. The suspension is cooled to −20° C. and70 μl of a 1.0M solution of diethyl (+)-tartrate (70 μmol) are addedthereto. The medium is left to stir under an argon atmosphere for 48hours at −20° C. An amount of 155 mg of propyloxiranemethanol (1.55mmol) is subsequently introduced and the combined mixture left to stirfor 30 minutes. An amount of 0.6 ml of a 6.0M solution of TBHP indichloromethane (3.6 mmol) is added and the solution is left to stirunder argon for 48 hours at −20° C. A propyloxiranemethanol yield of 31%and a degree of conversion of the trans-2-hexen-1-ol of 33%, thus aselectivity of 94%, for an enantiomeric excess of 90% (predominantly(S,S)-propyloxiranemethanol) are obtained.

After recycling the catalyst as in Example 13 and by carrying out thereaction under the same conditions as during the first use, apropyl-oxiranemethanol yield of 23% and a degree of conversion of thetrans-2-hexen-1-ol of 26%, thus a selectivity of 88%, for anenantiomeric excess of 90% (predominantly (S,S)-propyloxiranemethanol)are obtained.

Example 16

An amount of 675 mg of the solid prepared as indicated in Example 4-a,exhibiting a content by mass of tantalum of 0.67% (25 μmol of Ta), isplaced in a 50 ml round-bottomed flask under an argon atmosphere and 12ml of dichloromethane are added. The combined mixture is cooled to −20°C. and 28 μl of 1.0M solution of diethyl (+)-(R,R)-tartrate (28 μmol)are introduced. The suspension is left to stir at this temperature for15 hours, then 125 mg of trans-2-hexen-1-ol (1.25 mmol) are introducedand the mixture is left for 30 min before introducing 0.4 ml of a 6.5Msolution of TBHP (tBuOOH) in dichloromethane (approximately 2.6 mmol).The medium is left for 48 hours at −20° C. in order for the epoxidationreaction to take place. The medium is filtered through a sintered glassunder argon and the solid catalyst is washed four times withdichloromethane at ambient temperature and the solutions are combinedtogether. Analysis by GC gives a propyloxiranemethanol yield of 25%, aconversion of the trans-2-hexen-1-ol of 33%, i.e. a selectivity of 75%,and an enantiomeric excess of 90% (predominantly(S,S)-propyloxiranemethanol).

Example 17

An amount of 500 mg of the solid prepared in Example 4-b, exhibiting acontent by mass of tantalum of 5.40% (149 μmol of Ta), is placed in a100 ml round-bottomed flask with 40 ml of solvent (CH₂Cl₂). The combinedmixture is cooled to 0° C., 31 mg of diethyl (+)-tartrate (160 μmol) areintroduced and the mixture is left to stir at 0° C. for 15 hours. 400 mgof trans-2-hexen-1-ol (4.0 mmol) are then added and, 30 minutes later,1.6 ml of a 6.0M solution of TBHP in dichloromethane (9.6 mmol). Themedium is left for 48 hours at 0° C. in order for the epoxidationreaction to take place. The medium is filtered through a sintered glassunder argon, the solid catalyst is washed four times withdichloromethane at ambient temperature and the solutions are combinedtogether. Analysis by GC gives a propyloxiranemethanol yield of 14%, aconversion of the trans-2-hexen-1-ol of 25%, i.e. a selectivity of 56%,and an enantiomeric excess of 40% (predominantly(S,S)-propyloxiranemethanol).

Various results relating to the asymmetric epoxidation oftrans-2-hexen-1-ol to propyloxirane-methanol are presented in Table 2.

Conversion of Epoxy Selectivity % Enantiomeric M/allyl alcohol Reactionthe trans-2- alcohol for epoxy excess^(c) Rotation Catalyst^(a) (M = Taor Ti) conditions^(b) hexen-1-ol yield alcohol (predominantly)number^(d) Homogeneous 2/100 −20° C. 50% 23% 46% −55% (R, R) 11 Ta(OEt)₅(Example 14-b) (+)-DET with zeolites TBHP Sublimed [Ta] 2/100 −20° C.33% 25% 75% 85% (S, S) 13 0.67% (Example 16) (+)-DET without zeoliteTBHP Impregnated 4/100 −20° C. 35% 34% 97% 89% (S, S) 8.5 [Ta] 5.40%(Example 12) (+)-DET without zeolite TBHP Impregnated 4/100 −20° C. 48%40% 83% 90% (S, S) 10 [Ta] 5.40% (Example 14-a) (+)-DET with zeolitesTBHP Impregnated 4/100    0° C. 25% 14% 56% 40% (S, S) 3.5 [Ta] 5.40%(Example 17) (+)-DET without zeolite TBHP Sublimed 4/100 −20° C. 33% 31%94% 90% (S, S) 8 Ta(OEt)₅ (Example 15) (+)-DET 4.87% without zeoliteTBHP Homogeneous 5/100 −20° C. 99% 80% 81% 96% (S, S) 16 Ti(OiPr)₄(Example 14-c) (+)-DET with zeolites TBHP Key to Table 2 ^(a)Thecatalysts are prepared from tantalum compounds grafted to silica 500either by impregnation in pentane or by sublimation of the compoundunder vacuum. [Ta] denotes the compound ((CH₃)₃CCH₂)₃Ta═CHC(CH₃)₃. Twotests in a homogeneous medium are mentioned, from Ta(OEt)₅ andTi(OiPr)₄. ^(b)Reaction temperature - Chiral inductor ((+)-DET = DiEthyl(R,R)-Tartrate) - Presence or absence of zeolite - Oxidant (TBHP =tert-butyl hydroperoxide). ^(c)Enantiomeric excess (e.e.) = [(S,S) −(R,R)]/[(S,S) + (R,R)]; with (S,S): amount of(S,S)-propyloxiranemethanol and (R,R): amount of(R,R)-propyloxiranemethanol. ^(d)Rotation number, defined as above(Table 1)

Conclusion

With solid tantalum-based catalysts, the enantiomeric excesses are ofthe same order of magnitude as, indeed even greater than, those obtainedwith titanium in homogeneous catalysis for epoxide yields ranging up to80% and a selectivity close to 100%. It is clearly established that, inthe invention, the effect of the surface tantalum species is indeedcatalytic and not stoichiometric, since the calculation shows that at atemperature of 0° C., a tantalum atom can convert up to 500 molecules,indeed even more, of allyl alcohol to glycidol in 24 hours. The catalystis filtered off and is reused for a fresh catalytic test and similarresults are then obtained. The recycling can be carried out severaltimes without significant loss in activity or in stereoselectivity.Furthermore, it can be demonstrated that the tantalum does not pass intosolution and that the solid retains its same content of tantalum afterseveral recycling operations. Surprisingly, if the same experiment iscarried out with titanium complexes, a very low activity without anenantiomeric excess is then obtained.

It must be clearly understood that the invention refined by dependentclaims is not limited to the specific embodiments indicated in the abovedescription but encompasses alternative forms thereof which do notdepart either from the scope or the spirit of the present invention.

What is claimed is:
 1. A solid oxidation catalyst comprising a metalcompound of a pentavalent or hexavalent metal M, selected from the groupconsisting of tantalum, vanadium, niobium, chromium, molybdenum andtungsten, grafted to the surface of a solid oxide by at least onecovalent bond between an oxygen atom of the solid oxide and the metalatom M, the grafted metal compound exhibiting at least two alkoxy groupsbonded to the metal via an oxygen atom, with said at least two alkoxygroups bonded to the metal M belonging to a chiral polyol unit.
 2. Thecatalyst as claimed in claim 1, wherein the metal compound exhibits twoalkoxy groups which are bonded to the metal with an oxygen atom andwhich belong to a chiral diol unit.
 3. A catalyst as claimed in claim 2,characterized in that the diol unit is selected from the groupconsisting of: 1,2-propylene glycol 2,3-butanediol3,4-dimethyl-3,4-hexanediol 4,5-octanediol 2,3-hexanediol1,3-di(p-nitrophenyl)propane-1,2-diol 2,4-pentanediol tartaric acidesters, tartaric acid diamide N,N-dimethyltartaric acid diamidetrans-1,2-cyclopentanediol diethyl 1,2-cyclohexanediol-1,2-dicarboxylatedimethyl 2,4-dihydroxyglutarate ethyl N,N-diethyl tartrate monoamide2,5-dioxo-3,4-octanediol 1,2-bisacetylethylene glycolbis-2,2′-(2-hydroxycaprolactone) binaphthol, and1,2-bis(methoxyphenyl)ethane-1,2-diol.
 4. A catalyst as claimed in claim1, characterized in that the alkoxy groups of OR type bonded to themetal M via the oxygen atom are identical or different and in that the Rradicals are aliphatic or unsaturated, optionally cyclic, aromatic, C₁to C₃₀ hydrocarbonaceous chains which can optionally be functionalized.5. A catalyst as claimed in claim 4, characterized in that the R radicalis a C₁ to C₈, hydrocarbonaceous chain.
 6. A catalyst as claimed inclaim 4, characterized in that the R radicals are selected from methyl,ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, cyclohexyl,vinyl, allyl, phenyl or trialkylsilyl (R₃Si—; R=Me, Et, i-Pr or n-Bu).7. A catalyst as claimed in claim 1, characterized in that the metalcompound grafted to the solid oxide comprises 4 alkoxy groups where themetal M is selected from tantalum, vanadium or niobium and 4 or 5 alkoxygroups when the metal is selected from chromium, molybdenum or tungsten.8. A catalyst as claimed in claim 1, characterized in that the solidoxide is selected from the group consisting of silica, alumina,silica/alumina, zeolites, silicalites, titanium oxide, niobium oxide,tantalum oxide and mesoporous silicas.
 9. A process for the preparationof a solid oxidation catalyst, comprising bringing into contact a chiralpolyol and a catalyst precursor made of solid oxide comprising a metalcompound of a pentavalent or hexavalent metal M, selected from the groupconsisting of tantalum, vanadium, niobium, chromium, molybdenum andtungsten, grafted to the surface of a solid oxide by at least onecovalent bond between an oxygen atom of the solid oxide and the metalatom M, the grafted metal compound comprising at least two alkoxygroups, and exchanging at least two alkoxy groups of the grafted metalcompound with the polyol.
 10. The process according to claim 9, whereinthe chiral polyol is a chiral diol.
 11. The process as claimed in claim10, characterized in that the precursor catalyst and the chiral diol arereacted together in a solvent according to a diol:metal M molarproportion of at least 0.5.
 12. The process as claimed in claim 10,characterized in that the chiral diol used is selected from the groupconsisting of: 1,2-propylene glycol 2,3-butanediol3,4-dimethyl-3,4-hexanediol 4,5-octanediol 2,3-hexanediol1,3-di(p-nitrophenyl)propane-1,2-diol 2,4-pentanediol tartaric acidesters tartaric acid diamide N,N-dimethyl tartaric acid diamidetrans-1,2-cyclopentanediol diethyl 1,2-cyclohexanediol-1,2-dicarboxylatedimethyl 2,4-dihydroxyglutarate ethyl N,N-diethyl tartrate monoamide2,5-dioxo-3,4-octanediol 1,2-bisacetylethylene glycolbis-2,2′-(2-hydroxycaprolactone) binaphthol, and1,2-bis(methoxyphenyl)ethane-1,2-diol.
 13. The process as claimed inclaim 11, characterized in that the chiral diol used is selected fromthe group consisting of: 1,2-propylene glycol 2,3-butanediol3,4-dimethyl-3,4-hexanediol 4,5-octanediol 2,3-hexanediol1,3-di(p-nitrophenyl)propane-1,2-diol 2,4-pentanediol tartaric acidesters tartaric acid diamide N,N-dimethyl tartaric acid diamidetrans-1,2 cyclopentanediol diethyl 1,2-cyclohexanediol-1,2-dicarboxylatedimethyl 2,4-dihydroxyglutarate ethyl N,N-diethyl tartrate monoamide2,5-dioxo-3,4-octanediol 1,2-bisacetylethylene glycolbis-2,2′-(2-hydroxycaprolactone) binaphthol, and1,2-bis(methoxyphenyl)ethane-1,2-diol.
 14. The process as claimed inclaim 9, characterized in that the alkoxy groups of OR type bonded tothe metal M via the oxygen atom are identical or different and in thatthe R radicals are aliphatic or unsaturated, optionally cyclic C₁ to C₃₀hydrocarbonaceous chains which can optionally be functionalized.
 15. Theprocess as claimed in claim 14, characterized in that the R radical is aC₁ to C₈ hydrocarbonaceous chain.
 16. The process as claimed in claim14, characterized in that the R radicals are selected from the groupconsisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,tert-butyl, cyclohexyl, vinyl, allyl, phenyl, and trialkylsilyl (R₃Si—;R=Me, Et, i-Pr or n-Bu).
 17. The process as claimed in claim 9,characterized in that the metal compound grafted to the solid oxidecomprises 4 alkoxy groups where the metal M is selected from tantalum,vanadium or niobium; and 4 or 5 alkoxy groups when the metal is selectedfrom chromium, molybdenum or tungsten.
 18. The process as claimed inclaim 9, characterized in that the solid oxide is selected from thegroup consisting of: silica, alumina, silica/alumina, zeolites,silicalites, titanium oxide, niobium oxide, tantalum oxide andmesoporous silicas.
 19. A process for the oxidation of prochiralcompounds, comprising bringing into contact a prochiral compound, anoxidant, and a solid catalyst according to claim 1, and reacting saidprochiral compound, oxidant and solid catalyst together.
 20. The processaccording to claim 19 further comprising the asymmetric epoxidation ofprochiral olefinic double bonds of a compound to be epoxidized, saidcompound being a carbinol compound exhibiting an ethylenic double bondwhich is separated from the carbinol group by 0 to 1 C atom, in whichthe compound to be epoxidized, an oxidant, and a chiral solid catalystare brought into contact and are reacted together, said solid catalystcomprising a metal compound of a pentavalent or hexavalent metal M,selected from the group consisting of tantalum, vanadium, niobium,chromium, molybdenum and tungsten, grafted to the surface of a solidoxide by at least one covalent bond between an oxygen atom of the solidoxide and the metal atom M, the grafted metal compound exhibiting atleast two alkoxy groups bonded to the metal via an oxygen atom, withsaid at least two alkoxy groups bonded to the metal M belonging to achiral polyol unit.
 21. The process as claimed in claim 19,characterized in that, at the end of the reaction, a solid oxide isrecovered and is recycled.
 22. The process as claimed in claim 21,wherein the solid oxide is a silica dehydrated between 200 and 800° C.23. The process as claimed in claim 22, wherein the silica is treatedwith an organosilicon compound.